In my experience, if you need to divert at your destination , the route given by ATC is pretty ad hoc anyway.
Most of the time they‘ll throw some waypoints at you and you need to set up the flight plan on the go, including negotiation of the runway at the alternate airport.
Imho not much to set up already before takeoff.
preplanned sec flpn comes in more handy for having one engine out departures or immediate returns to the departure airport ready at hand and preplanned.
Other than that, when diverting, enter a hold at your actual destination (active fpln), copy diversion route instructions from ATC and set up your diversion route in sec fpln.
When done, activate sec fpln, and direct-to first waypoint (or whatever atc wants).
When not holding, e. g. direct diversion right after go-around, same thing. Be on HDG, prepare/activate sec fpln, then direct-to first wpt, and back to NAV.
Surely there are many ways to skin a cat. I found diversions to be highly dynamic in nature. Usually ATC asks for intentions after GA and will accomodate.